Metal oxide varistors of the type consisting of zinc oxide doped with various metal compounds find application as voltage surge protective devices for protecting electrical equipment during overvoltage conditions. The varistor discs are generally provided with metallic electrodes on the large diameter opposing surfaces of the disc and with an insulating collar around the disc periphery. The electrodes provide electrical connection with the zinc oxide material and the insulating collar around the periphery prevents the varistor current from short circuiting across the edges of the disc. The discs are usually encapsulated in a silicone rubber compound to aid in dissipation of heat from the discs resulting from the absorption of surge energy.
During the varistor manufacture, care is taken to ensure that the chemistry of the zinc oxide material is as uniform as possible throughout the entire disc structure. This is done to provide reasonably uniform current flow and low power loss through the cross section of the disc, and to provide uniform properties among individual varistors made from the same batch of zinc oxide material.
Varistors discs, after pressing and sintering, are lapped to make the large diameter surfaces flat and parallel to each other. The discs are then etched in a nitric acid solution to provide surfaces suitable for the attachment of metallic electrodes. After the electrodes are attached, a ceramic insulating collar is applied. A silicone rubber compound is usually applied over the ceramic insulating collar for heat dissipation purposes. The use of the heat dissipation material is fully described in U.S. patent applications Ser. No. 778,006 and Ser. No. 778,007, now U.S. Pat. Nos. 4,092,694 and 4,100,588, respectively.
It has been discovered that the chemical and electrical properties of the outer surfaces of the discs are different from the bulk properties of the discs. This appears to result from the disc surfaces being directly exposed to the atmosphere during the pressing and sintering operations; furthermore, surface contamination can occur during the chemical etching process used prior to application of the electrodes and insulating collar. It has been found as a result of the difference in surface properties when the silicone rubber heat dissipation material is applied, that the watts loss of the varistor under applied AC voltage stress may increase considerably over the watts loss measured at the same stress before the heat dissipation material is applied.
One purpose of this invention is to treat the zinc oxide disc prior to depositing the insulating collar in order to allow the heat dissipation material to be applied without an increase in watts loss.